1. Field of the Invention
This invention relates to apparatus for making a yarn, comprising two juxtaposed, closely spaced apart twisting drums, which rotate in the same sense and between themselves define a generally triangular twisting space, to which a vacuum is applied, fiber-feeding means and withdrawing means for withdrawing the yard which has been formed in that the fibers feb to said twisting space have been twisted together in said twisting space.
2. Description of the Prior Art
When the fibers of a drawn roving have been twisted together between two twisting drums rotating in the same sense, an untwisting of the fibers in a false-twisting sense should be avoided and a proper cohesion of the fibrous structure should be ensured. This is accomplished in that covering fibers are wound about the drawn roving as it is twisted. During the winding of said covering fibers, the fact that the covering fibers have been parallelized by means of a drawing frame and that they are constrained to move on a feeding drum to which a vacuum is applied in a portion of the periphery of the drum should be utilized for the making of a uniform yarn having a high strength. From Austrian Patent Specification No. 377,018 it is known that this can be accomplished in that the drum used to feed the covering fibers, which drum is coaxial to one of the twisting drums, is driven at a surface speed which differs from that of the twisting drums so that the higher surface speed of the twisting drums which is required for the twisting of the drawn roving will not adversely affect the uniform winding of the covering fibers about the drawn roving. Whereas that control of the direction in which the covering fibers are fed into the triangular twisting space is an essential requirement for the making of uniform yarns of high strength, the quality of the yarn will also depend on a proper tying of the covering fibers into the fibrous structure formed from the drawn roving as it is twisted. The covering fibers can be more effectively tied into the fibrous structure when the covering fibers fly freely into the twisting space. Besides, the provision of a drawing frame for the roving to be covered and of an additional drawing frame for the covering fibers will involve a relatively high structural expenditure. Moreover, the roving from which the covering fibers are derived cannot readily be drawn to any high degree which may be desired so that yarns, particularly if they are thin, must be expected to contain a relatively high proportion of covering fibers.